Solid golf balls are usually two or more piece constructions. Two-piece golf balls include a single-piece core and a cover. The core forms a golf ball subassembly that the cover surrounds. Three-piece golf balls include a core, an intermediate layer, and a cover. In such balls, the core and intermediate layer form the golf ball subassembly that the cover surrounds.
For a preferred cover the material is a thermosetting polyurethane. One method of making polyurethane-covered golf balls includes disposing the golf ball subassembly into a cover mold and casting the cover thereon. During casting, heat is generated by an exothermic reaction of the thermoset processes. As a result of this heat, the ball subassembly tends to undergo volumetric thermal expansion. The thermal expansion of the subassembly can force the cover mold open and cause the subassembly to shift in the mold so that the cover is uneven and has excessive flash. This can result in an unplayable ball.
Prior solid golf balls having cast urethane covers were made using a method that includes preheating the golf ball subassembly to a predetermined elevated temperature. Preheating the subassembly is done to the extent which causes the subassembly to undergo volumetric thermal expansion. Thereafter, the cover is cast onto the subassembly. Preheating decreases the total temperature change the subassembly is exposed to and minimizes the thermal expansion of the subassembly in the cover mold.
There are several drawbacks to this method. First, it requires that the tooling be sized to accommodate expansion due to preheating rather than for the actual subassembly size. This method also requires that the subassembly expands beyond its ambient dimensions. This method also requires that the subassembly go through large extremes of temperature from pre-heating to cooling. In this process, the subassembly is very sensitive to temperature and any differences in temperature can effect the cover properties. Consequently, the golf ball product can vary in size undesirably when it encounters different temperatures.
Similar problems may arise with materials other than thermoset polyurethane using processes other than casting. It is desirable to provide a method of making a golf ball that overcomes these limitations and produces the improved, playable golf balls of the present invention.